1. Field of the Invention
The present invention relates to a voltage-frequency conversion apparatus and a method of generating a reference voltage of the voltage-frequency conversion apparatus, which are suitable for detecting the remaining voltage of electricity charged in a secondary battery, for example.
2. Description of the Related Art
==Configuration of Conventional Voltage-Frequency Conversion Apparatus==
<Overall Configuration>
With reference to FIG. 3, an example configuration of a conventional voltage-frequency conversion apparatus is described. FIG. 3 is a circuit block diagram showing the example configuration of the conventional voltage-frequency conversion apparatus.
A voltage-frequency conversion apparatus 100 shown in FIG. 3 has an error amplifier 102, a variable current source 104, a reference voltage source 106, a comparator 108, a capacitor 110, a switch element 112 and a control logic circuit 114.
The error amplifier 102 is applied with a voltage VIN(+) and a voltage VIN(−) and generates an output voltage corresponding to a differential voltage (an error) between the voltage VIN(+) and the voltage VIN(−). In other words, the greater the difference between the voltage VIN(+) and the voltage VIN(−) is, the larger the output voltage generated by the error amplifier 102 becomes.
A current amount generated by the variable current source 104 is controlled according to the output voltage of the error amplifier 102. In other words, the larger the output voltage of the error amplifier 102 is, the larger the current generated by the variable current source 104 becomes. The variable current source 104 and the capacitor 110 are serially connected between a power source VDD and ground, and the capacitor 110 is charged with the current generated by the variable current source 104. In other words, the larger the current generated by the variable current source 104 is, the faster the capacitor 110 is charged and, on the other hand, the smaller the current generated by the variable current source 104 is, the slower the capacitor 110 is charged.
The comparator 108 compares a charging voltage occurring at one end of the capacitor 110 on the non-grounded side and a constant reference voltage VREF generated by the reference voltage source 106. In FIG. 3, the charging voltage of the capacitor 110 is applied to a plus (non-inverting input) terminal of the comparator 108 and the reference voltage VREF is applied to a minus (inverting input) terminal of the comparator 108. Therefore, the comparator 108 outputs a low level if the charging voltage of the capacitor 110 is smaller than the reference voltage VREF and outputs a high level if the charging voltage of the capacitor 110 exceeds the reference voltage VREF. In other words, the comparator 108 outputs a frequency signal corresponding to the differential voltage between the voltage VIN(+) and the voltage VIN(−).
The switch element 112 is connected in parallel with the capacitor 110. A bipolar transistor, a MOSFET, etc. can be employed as the switch element 112.
The control logic circuit 114 is connected to the output of the comparator 108 and controls the turning on/off of the switch element 112. In other words, the control logic circuit 114 renders the switch element 112 on for a certain period after the output of the comparator becomes a high level. In this period, the capacitor 110 is discharged through the switch element 112.
==Operation of Conventional Voltage-Frequency Conversion Apparatus==
With reference to FIG. 3 and FIG. 6, the operation of the voltage-frequency conversion apparatus 100 is described. FIG. 6 is a waveform diagram showing a relationship between the charging voltage appearing at one end of the capacitor 110 and a frequency signal output from the comparator 108. A rate (gradient) of increase in the charging voltage of the capacitor 110 varies depending on the amount of the current supplied from the variable current source 104. In other words, as the current supplied from the variable current source 104 becomes smaller, the gradient of increase in the charging voltage of the capacitor 110 changes from a dashed line to a solid line to a dot-and-dash line in the direction of becoming more gradual. Frequency signals A, B, C are signals generated respectively corresponding to the charging voltages indicated by the dashed line, the solid line and the dot-and-dash line.
If the differential voltage between the voltage VIN(+) and the voltage VIN(−) equals VA, the variable current source 104 generates a current IA and the capacitor 110 is supplied with the current IA and charged as shown by the dashed line. If the charging voltage of the capacitor 110 is smaller than the reference voltage VREF, the output of the comparator 108 is at a low level. If the charging voltage of the capacitor 110 exceeds the reference voltage VREF subsequently, the output of the comparator 108 becomes a high level. The control logic circuit 114 renders the switch element 112 on for a certain period after the output of the comparator 108 becomes the high level. In other words, a discharge path is formed for the capacitor 110. Therefore, the capacitor 110 immediately discharges via the switch element 112, as shown by the dashed line. The certain period in which the control logic circuit 114 renders the switch element 112 on is a time period required for completing the discharge of the capacitor 110 and is decided in consideration of the capacity of the capacitor 110, etc. and preset in the control logic circuit 114. If the charging voltage of the capacitor 110 becomes smaller than the reference voltage VREF, the output of the comparator 108 becomes the low level again. Thus, the comparator 108 outputs the frequency signal A in response to the charging voltage indicated by the dashed line.
If the differential voltage between the voltage VIN(+) and the voltage VIN(−) equals VB (<VA), the variable current source 104 generates a current IB (<IA) and the capacitor 110 is supplied with the current IB and charged as shown by the solid line. If the charging voltage of the capacitor 110 is smaller than the reference voltage VREF, the output of the comparator 108 is at the low level. If the charging voltage of the capacitor 110 exceeds the reference voltage VREF subsequently, the output of the comparator 108 becomes the high level. The control logic circuit 114 renders the switch element 112 on for a certain period after the output of the comparator 108 becomes the high level. Therefore, the capacitor 110 immediately discharges via the switch element 112, as shown by the solid line. If the charging voltage of the capacitor 110 becomes smaller than the reference voltage VREF, the output of the comparator 108 becomes the low level again. Thus, the comparator 108 outputs the frequency signal B in response to the charging voltage indicated by the solid line.
If the differential voltage between the voltage VIN(+) and the voltage VIN(−) equals VC (<VB), the variable current source 104 generates a current IC (<IB) and the capacitor 110 is supplied with the current IC and charged as shown by the dot-and-dash line. If the charging voltage of the capacitor 110 is smaller than the reference voltage VREF, the output of the comparator 108 is at a low level. If the charging voltage of the capacitor 110 exceeds the reference voltage VREF subsequently, the output of the comparator 108 becomes the high level. The control logic circuit 114 renders the switch element 112 on for a certain period after the output of the comparator 108 becomes the high level. Therefore, the capacitor 110 immediately discharges via the switch element 112, as shown by the dot-and-dash line. If the charging voltage of the capacitor 110 becomes smaller than the reference voltage VREF, the output of the comparator 108 becomes the low level again. Thus, the comparator 108 outputs the frequency signal C in response to the charging voltage indicated by the dot-and-dash line.
As described above, the voltage-frequency conversion apparatus 100 converts the differential voltage between the voltage VIN(+) and the voltage VIN(−) to a frequency signal corresponding to the differential voltage.
==Example Application of Voltage VIN(+) and Voltage VIN(−)==
The voltage-frequency conversion apparatus 100 can be employed as an apparatus for determining the remaining voltage of electricity charged in a secondary battery for example.
FIG. 4 is a schematic configuration diagram showing a battery pack incorporating a secondary battery. In FIG. 4, a battery pack 200 incorporates a secondary battery 201, a sensing resistor 202, a microcomputer 203 (or a logic integrated circuit), etc. The secondary battery 201 and the sensing resistor 202 are serially connected between a plus terminal and a minus terminal to be electrically connected to an electronic device using the secondary battery 201 as a power source. When the secondary battery 201 is charged or discharged, the sensing resistor 202 produces the voltage VIN(+) and the voltage VIN(−) at both its ends. For example, if the battery pack 200 is mounted in an electronic device, the secondary battery 201 discharges to supply power to the electronic device and a discharging current flows in an a direction (upwards on the page) through the sensing resistor 202. In other words, when the secondary battery 201 discharges, the voltage VIN(+) becomes lower than the voltage VIN(−). The smaller the discharging amount of the second battery 201 is, the greater the differential voltage between the voltage VIN(+) and the voltage VIN(−) becomes. On the other hand, if the battery pack 200 is mounted on a charger (not shown), the secondary battery 201 is charged and a charging current flows in a b-direction (downwards on the page) through the sensing resistor 202. In other words, when the secondary battery 201 is charged, the voltage VIN(+) becomes higher than the voltage VIN(−). The larger the charging amount of the second battery 201 is, the greater the differential voltage between the voltage VIN(+) and the voltage VIN(−) becomes.
The voltage VIN(+) and the voltage VIN(−) are supplied to the microcomputer 203 as voltage information which is the basis for determining the remaining voltage of electricity when the secondary battery 201 discharges or the charging voltage when the secondary battery 201 is charged. The microcomputer 203 incorporates the voltage-frequency conversion apparatus 100. Hence, the microcomputer 203 can obtain a frequency signal corresponding to the levels of the voltage VIN(+) and the voltage VIN(−) as well as the differential voltage between the voltage VIN(+) and the voltage VIN(−). The microcomputer 203 performs appropriate program processing on the obtained frequency signal, thereby obtaining the remaining voltage of electricity and the usable time for the remaining voltage of electricity when the secondary battery 201 is mounted on an electronic device, and the charging voltage while being charged (see, e.g., Japanese Patent Application Laid-Open Publication No. 2002-107428).
FIG. 5 is a graph showing an input-output characteristic of a voltage-frequency conversion apparatus. In FIG. 5, the horizontal axis shows a differential voltage [V] between the voltage VIN(+) and the voltage VIN(−) input into the error amplifier 102 and the vertical axis shows the frequency [Hz] of the frequency signal output from the comparator 108. The differential voltage [V] between the voltage VIN(+) and the voltage VIN(−) and the frequency signal ideally has proportionality as shown by the solid line.
By the way, the microcomputer 203 (or a logic integrated circuit) executes calculation operation in synchronization with its own or external clock signal. In the case of FIG. 4, the voltage-frequency conversion apparatus 100 is incorporated in the microcomputer 203 to operate, as described above. Therefore, if a predetermined relationship exists between the frequency of the frequency signal output by the comparator 108 in the voltage-frequency conversion apparatus 100 and the frequency of the clock signal used by the microcomputer 203, the former frequency signal receives interference (digital noises) from the latter clock signal. The predetermined relationship is, for example, that the frequency of the former frequency signal is an integral multiple of the frequency of the latter clock signal. This can happen because the frequency of the frequency signal is a unique constant frequency depending on a charging voltage of the capacitor 110. If the former frequency signal is subject to interference from the latter clock signal, the frequency of the frequency signal output by the comparator 108 becomes constant without responding to the change in the differential voltage between the voltage VIN(+) and the voltage VIN(−). In other words, if being subject to interference from the clock signal, the voltage-frequency conversion apparatus 100 has a dead zone where the change in the frequency signal is constrained. As a result, the problem occurs that the actual input-output characteristic of the voltage-frequency conversion apparatus 100 deviates from the real input-output characteristic indicated by the solid line to one indicated by a dot-and-dash line.
The problem of interference between the voltage-frequency conversion apparatus 100 and the clock signal occurs not only in the case that the voltage-frequency conversion apparatus 100 is incorporated in the microcomputer 203, the logic integrated circuit, etc., but for example, even if the voltage-frequency conversion apparatus 100 and a circuit using the clock signal having a predetermined frequency such as an integrated circuit or a discreet circuit are provided separately, the interference problem may occur in a design environment where the clock signal is a disturbance factor for the voltage-frequency conversion apparatus 100.
If the voltage-frequency conversion apparatus 100 has the input-output characteristic as shown by the dot-and-dash line of FIG. 5, the microcomputer 203 obtains a remaining voltage of electricity greater than a real remaining voltage of electricity as the remaining voltage of the secondary battery 201, for example, where the battery pack 200 is mounted in an electronic device. In this case, if the microcomputer 203 displays the remaining voltage of the secondary battery 201 on a display of the electronic device, the remaining voltage of the secondary battery 201 may suddenly become zero when the display status is suggesting that the remaining voltage of the secondary battery 201 is sufficient, resulting in a shutdown of the electronic device, which gives a user so much trouble.